Hormone replacement therapy reverses the decline in stroke volume evident in postmenopausal women. Whether this effect is related to changes in blood volume or to a direct myocardial effect is unknown. Accordingly, we examined individual hormone replacement components, 17 - estradiol (E, and estrogen) and medroxyprogesterone acetate (P, a progestin), for their effect on cardiac function by 2-dimensional (2-D) and Doppler echocardiography, and correlated these findings to measures of plasma and blood volume. Eighteen healthy postmenopausal women received P (Provera, 5 mg per day) for 2 months, followed by 2 months of sequential E (Estrace, 1 mg per day) with Provera 10 mg/day for the last 12 days of each month. Volunteers had echocardiograms before starting hormones, after 8 weeks of P, after 14 weeks during E, and after 16 weeks during E + P. Plasma and blood volume were measured using 125I-albumin dilution at baseline and during one of the other 3 time points. P alone increased end diastolic volume (EDV) by 4.5%, stroke volume (SV) by 6.6%, and left ventricular mass (LVM) by 5% over baseline (all p<0.05), but had no significant effect on cardiac output (CO). During E treatment, CO increased by 10%, SV by 10%, and LVM by 6.5% over baseline (all p<0.05). Combination E + P produced the largest changes, with a 12.8% increase in CO, 11.7% increase in SV, 7.2% increase in EDV, 10.2% decrease in end systolic volume (ESV), 10.9% increase in ejection fraction (EF), and a 9.4% increase in LVM (all p<0.05). There was a significant increase in blood volume (5.2%) and plasma volume (4.8%) from baseline during treatment (p<.001 and <.01, respectively), which could explain the increased cardiac output but not the increased ejection fraction. In conclusion, combination hormone replacement therapy results in modest but significant increases in cardiac output and left ventricular ejection fraction. Taken together, these data support a direct myocardial effect of hormone replacement that is preload-independent.